This invention relates to the design of optical coatings for controlling the manner in which light of particular wavelengths is transmitted by or reflected from an optical surface.
The phenomenon of optical interference, which causes modifications in the transmitted and reflected intensities of light, occurs when two or more beams of light are combined. The principle of superposition states that the resultant amplitude is the complex sum of the amplitudes of the individual beams. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves, which are created by reflection from the upper and lower surfaces of the thin film of soap solution or oil.
One important practical application for these interference effects in thin films involves the production of coated optical surfaces. If a film of a transparent substance is deposited on glass, for example, with a refractive index which is properly specified relative to the refractive index of the glass and with a thickness which is one quarter of a particular wavelength of light in the film, the reflection of that wavelength of light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a nonreflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the antireflective performance of such films by using a composite film having two or more superimposed layers. In theory, it is possible with this approach to design a wide range of multiple-layer interference coatings for obtaining a great variety of transmission and reflection spectrums. This has led to the development of many new optical devices which incorporate complex spectral filter structures. Antireflection coatings, laser dielectric mirrors, television camera edge filters, bandpass filters, and stopband filters are some of the useful devices employing such thin-film interference coatings.
Frequently two different materials are used in fabricating such a composite film, one with a relatively high index of refraction and a second material with a relatively low index of refraction. In the digital approach (see, e.g., Southwell, U.S. Pat. No. 4,666,250), the two materials are alternately deposited to specified thicknesses to obtain the desired optical characteristics for the film. Some advanced applications of optical technology, however, require antireflective films which adhere more closely to theoretically specified refractive index profiles. These advanced applications can be achieved through the use of discrete layers having intermediate values of refractive index, which requires the coevaporation of two materials, or the use of a gradient index coating, in which the index of refraction within the coating varies continuously as a function of depth.
Where an application requires an optical coating which controls the spectral qualities of the transmitted and reflected light, such as allowing only wavelengths within a specified band of wavelengths to pass through the coating, it is generally desirable to apply an antireflecting coating to the filter, because the antireflecting coating can improve the performance of the filter over a broad spectral range. U.S. Pat. No. 4,583,822 to Southwell, for example, discloses one such antireflective design for a thin film optical coating. Unfortunately, however, such an antireflecting coating also may degrade the performance of the spectral filter within a narrow wavelength range of interest. Thus it would be desirable to provide the desirable antireflecting features of an antireflecting coating in a thin film spectral filter design while avoiding the concomitant degradation in performance which has accompanied the addition of antireflecting layers in the prior art.